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Recommendations for DNSSEC Resolvers Operators

Document Type Active Internet-Draft (dnsop WG)
Authors Daniel Migault , Dan York
Last updated 2022-06-07 (Latest revision 2022-05-13)
Replaces draft-mglt-dnsop-dnssec-validator-requirements
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dnsop                                                         D. Migault
Internet-Draft                                                  Ericsson
Intended status: Informational                                   D. York
Expires: 14 November 2022                                           ISOC
                                                             13 May 2022

             Recommendations for DNSSEC Resolvers Operators


   The DNS Security Extensions (DNSSEC) define a process for validating
   received data and assert them authentic and complete as opposed to

   This document clarifies the scope and responsibilities of DNSSEC
   Resolver Operators (DRO) as well as operational recommendations that
   DNSSEC validators operators SHOULD put in place in order to implement
   sufficient trust that makes DNSSEC validation output accurate.  The
   recommendations described in this document include, provisioning
   mechanisms as well as monitoring and management mechanisms.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 14 November 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.

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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  DNSSEC Validator Description  . . . . . . . . . . . . . . . .   5
   5.  Recommendations Categories  . . . . . . . . . . . . . . . . .   7
   6.  Time deviation and absence of Real Time Clock
           Recommendations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Trust Anchor Related Recommendations  . . . . . . . . . . . .   8
     7.1.  Trust Anchor Configuration  . . . . . . . . . . . . . . .   9
       7.1.1.  Definition of the Trust model . . . . . . . . . . . .  10
       7.1.2.  Trust Model Bootstrapping . . . . . . . . . . . . . .  11
       7.1.3.  Configuration Generation  . . . . . . . . . . . . . .  12
       7.1.4.  DNSSEC Resolver Instantiation . . . . . . . . . . . .  12
     7.2.  Trust Anchor Update . . . . . . . . . . . . . . . . . . .  12
       7.2.1.  Automated Updates to DNSSEC Trust Anchors . . . . . .  13
       7.2.2.  Automated Trust Anchor Check  . . . . . . . . . . . .  13
   8.  Negative Trust Anchors Related Recommendations  . . . . . . .  15
   9.  ZSK / KSK (non TA) Related Recommendations  . . . . . . . . .  16
   10. DNSKEY Related Recommendations  . . . . . . . . . . . . . . .  18
     10.1.  Automated Reporting  . . . . . . . . . . . . . . . . . .  18
     10.2.  Interactions with the cached RRsets  . . . . . . . . . .  18
   11. Cryptography Deprecation Recommendations  . . . . . . . . . .  19
   12. Invalid Reporting Recommendations . . . . . . . . . . . . . .  19
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  20
   15. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  21
   16. Acknowledgment  . . . . . . . . . . . . . . . . . . . . . . .  21
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     17.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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2.  Introduction

   The purpose of DNSSEC Resolver Operator (DRO) is to enable DNSSEC
   validation in their resolvers.

   By validating with DNSSEC a received Resource Record Set (RRset), the
   resolver provides a high level of confidentiality that the
   information carried by the RRset is effectively the one published by
   the legitimate owner of the RRset.  The act of DNSSEC validation
   [RFC4033][RFC4035] can be broken into two part: A _Signature
   Validation_ which binds the RRset to a private key as well as _Trust_
   in the owner of the private being the legitimate owner.

   Signature Validation consists in checking the cryptographic signature
   of a RRset and involves among other parameters a DNSKEY Resource
   Record (RR) and RRSIG RR and the RRset itself.  The signature
   validation process results in designating the owner of RRset as the
   owner of the corresponding private part of the public key contained
   in the DNSKEY RR.  Cryptography provides a high level of confidence
   in the binding to the private key.  In that sense, a rogue RRset
   likely results from the private key being exposed or guessed - as
   opposed to signature or key collisions for example.  As such differ
   the confidence into the Trust to designate which DNSKEY RR is

   Trust implicitly assumes the private keys used to signed are not
   shared and as such can be associated their respective owners.  The
   purpose of Trust is to put significant confidence into designating
   the legitimate DNSKEY RR - which also characterizes the corresponding
   private key.  Such trust is provided by a Trust Anchor (TA), and the
   chain of trust established between the TA and the DNSKEY RR.  The
   chain of trust is obtained by recursively validating the DNSKEY RRs.

   As a result, such trust results from the trust placed in the TA as
   well as the delegation mechanism provided by DNSSEC and the Signature
   Validation.  As TAs need to be managed over time, the trust also
   concerns the management procedure of the TA.  This is the main
   concern of this document.

   Data's authenticity and integrity is tied to the operator of the key
   that generates the signature.  It is conceivable that a validator
   could "know" the keys of each data source, but this is not practical
   at large scale.  To counter this, DNSSEC relied on securely chaining
   keys in a manner isomorphic to the way names are delegated.  Keys for
   a name will "vouch for" keys at a name delegated via the signing of a
   DS resource record set.

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   Using keys to vouch for keys, recursively, works when a manageable
   set of key to name associations are determined to be "trusted" - and
   are called trust anchors.  In DNSSEC, a validator needs one or more
   Trust Anchors from which to grow chains of verified keys.

   With operational experience, a twist has emerged.  More often, to
   date, failed validation is due to operator error or software bug
   [ENT] and not an attempt to forge data.  In general badly signed
   RRsets or zone badly delegated are out of scope of the DRO's
   responsibility.  However, the DRO may reflect this operational error
   with a temporary solution designated as Negative Trust Anchors (NTA)
   [RFC7646].  A NTA instructs a validator to ignore the presence of
   keys for a name, reacting as if the name is unsigned.

   Once accurately validated the RRset is assumed to be accurately
   validated and trusted during the time indicated by its TTL.

   The responsibilities of a DRO are limited to the management of TAs as
   well as providing the necessary infrastructure to perform the
   signature validation, e.g. appropriated libraries and time.  More
   specifically, badly signed zones or insertion of malicious DNSKEY
   fall out of the DRO's responsibilities.  Even though these threats
   fall out of these responsibilities, a DRO may collaborate with
   authoritative servers to limit the damage of their operational

   This document is focused on operational recommendations that DRO
   SHOULD put in place in order to implement sufficient trust that makes
   DNSSEC validation output accurate.  The recommendations described in
   this document include, provisioning mechanisms as well as monitoring
   and management mechanisms.

   The mechanisms provided are designed in accordance of the DNSSEC
   trust model as to meet the current operations of DNSSEC.  Such trust
   model is briefly recapped in Section 4 so operators understand the
   limits and motivations for such mechanisms.

3.  Terminology

   This document uses the following terminology:

   DNSSEC validator  the entity that performs DNSSEC resolution and
      performs signature validation.

   Accurate validation  validation that avoids false positives and
      catches true negatives.

   Trust Anchor Data Store  a module (of code) implementing functions

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      related to the trust anchors used by the validator.  This is
      essentially a database allowing access, monitoring of, and changes
      to trust anchors.

   DNSSEC Resolver Operator (DRO)  The operator providing DNSSEC
      validation service and managing DNSSEC validators

4.  DNSSEC Validator Description

   This is a conceptual block diagram of the elements involved with
   DNSSEC validation.  This is not meant to be an architecture for code,
   this is meant to be a framework for discussion and explanation.

       +-------------+  +---------------+
       |             |  |               |
       | Time Source |  | Cryptographic |
       |             |  |   Libraries   |
       |             |  |               |
       +-------------+  +---------------+
              |                 |
              v                 v
       +--------------------------------+   +--------------+
       |                                |   |              |
       |                                |<--| Trust Anchor |
       |    DNSSEC Validation Engine    |   |   Manager &  |
       |                                |-->|   Storage    |
       |                                |   |              |
       +--------------------------------+   +--------------+
             ^ |               ^                   |
             | v               |                   |
       +-------------+  +---------------+          |
       |             |  |               |          |
       | DNS Caches  |  | DNS Messages  |<---------+
       |             |  |               |
       +-------------+  +---------------+

   { title="DNSSEC Validator Description" }

   Time Source  The wall clock time provides the DNSSEC Validation
      Engine the current time.  Time is among other used to validate the
      RRSIG Signature and Inception Fields to provide some protection
      against replay attacks.

   Cryptograhic Libraries  The code performing mathematical functions
      provides the DNSSEC Validation Engine the ability to check the
      Signature Field that contains the cryptographic signature covering
      the RRSIG RDATA.

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   DNS Message  DNS responses are used to carry the information from the
      DNS system.  The receiver of the DNS message can be any kind of
      application including DNS-related application such as in the case
      of automated Trust Anchor update performed by the Trust Anchor
      Manager & Storage.  The DNSSEC Validator Engine accurately
      validates the DNS responses before caching them in the DNS Cache
      and forwarding them to the DNS receiver.  In case of validation
      failure, an error is returned and the information may be
      negatively cached.

   DNS Caches  Include positive and negative caches.  The DNSSEC
      Validation Engine fills DNS Caches with the results of a
      validation (positive data, negative failures).  The DNSSEC trust
      model considers that once a RRset has been accurately validated by
      the DNSSEC Validator Engine, the RRset is considered trusted (or
      untrusted) for its associated TTL.  DNS Caches contain RRsets that
      may contain information requested by the application (RRset of
      type AAAA for example) as well as RRset necessary to accurately
      validate the RRsets (RRsets of type DNSKEY or RRSIG for example).
      It also worth noticing that RRset validated with DNSSEC or RRset
      that are not validated with DNSSEC fill the DNS Cache with the
      same level of trust.

   Trust Anchor Manager  The database of trust anchors associated to
      database management processes.  This function provides the DNSSEC
      Validation Engine Trust Anchor information when needed.  When TA
      needs to be updated, the Trust Anchor Manager is also responsible
      to handle the updating procedure.  This includes sending DNS
      Messages as well as treating appropriately the DNS responses that
      have been accurately validated by the DNSSEC Validator Engine.

      This will require the DNSSEC Validator to update Trust Anchor
      information, whether via methods like Automated Updates of DNSSEC
      Trust Anchors [RFC5011], management of Negative Trust Anchors, or
      other, possibly not yet defined, means.

   DNSSEC Validation Engine  follows local policy to approve data.  The
      approved data is returned to the requesting application as well as
      in the DNS Caches.  While the cryptographic computation of the
      RRSIG signature may be the most visible step, the RRSIG record
      also contains other information intended to help the validator
      perform its work, in some cases "sane value" checks are performed.
      For instance, the original TTL (needed to prepare the RR set for
      validation) ought to be equal to or higher than the received TTL.

   Not shown - Name Server Process Management interfaces to elements,
   handling of Checking Disabled request, responses, as well as all API
   requests made of the name server.

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5.  Recommendations Categories

   DRO needs to be able to enable DNSSEC validation with sufficient
   confidence they will not be held responsible in case their resolver
   does not validate the DNSSEC response.  The minimization of these
   risks is provided by setting automated procedures, when a resolver is
   started or while it is operating, as well as some on-demand
   operations that enable the DRO to perform a specific operation.  The
   recommendations do not come with the same level of requirements and
   some are likely to be required.  Other are optional and may be
   followed by more advanced DROs.

   The recommendations are voluntary restrictive to prevent side effects
   of interventions that will be hard to predict, debug and understand.

   The RECOMMENDATIONS in this document are subdivided into the
   following categories:

   Start-up recommendations  which describes RECOMMENDED operations the
      DRO is expected to perform when the resolver is started.  These
      operations typically includes health check of the infrastructure
      the resolver is instantiated on as well as configuration check.

   Run time recommendations  which describes RECOMMENDED operations the
      DRO is expected to perform on its running resolvers.  These
      operations typically include health checks of the infrastructure
      as well as the resolvers.

   On demand recommendations  which describes the RECOMMENDED operations
      a DRO may perform.  This includes the ability to operate health
      check at a given time as well as specific operations such as
      flushing the cache.  The reason the document mentions these
      recommendations is to enable DROs to have the appropriates tools
      as well as to restrict their potential interventions.

6.  Time deviation and absence of Real Time Clock Recommendations

   With M2M communication some devices are not expected to embed Real
   Time Clock (Raspberry Pi is one example of such devices).  When these
   devices are re-plugged the initial time is set to January 1 1970.
   Other devices that have clocks that may suffer from time deviation.
   These devices cannot rely on their time estimation to perform DNSSEC

   Time synchronization may be performed manually, but for the sake of
   operations it is strongly RECOMMENDED to automate the time
   synchronization on each resolver.

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   *  DRO MUST provide means to update the time without relying on
      DNSSEC when the DNSSEC validator is started.  The resolver MUST
      NOT start if the time synchronization does not succeed at start

   Note that updating time in order to be able to perform DNSSEC
   validation may become a form of a chicken-and-egg problem when the
   NTP server is designated by its FQDN.  The update mechanisms must
   consider the DNSSEC validator may not able to validate the DNSSEC
   queries.  In other words, the mechanisms may have to update the time
   over an unsecure DNSSEC resolution.


   *  While operating, DRO MUST closely monitor time derivations of the
      resolvers and maintain the time synchronized.


   *  A DRO SHOULD be able to check and synchronize, on demand, the time
      of the system of its resolver.

   Note that time synchronization is a sensible operation and DRO MUST
   update the time of the systems over an authenticated and secure

   For all recommendations, it is strongly RECOMMENDED that
   recommendations are supported by automated processes.

7.  Trust Anchor Related Recommendations

   A TA store maintains associations between domain names and keys
   (whether stored as in a DNSKEY resource record or a DS resource
   record) and domain names whose key are to be ignored (negative trust
   anchors).  The TA store is essentially a database, storing the
   (positive) trust anchors.  Management of the TA can be done manually
   or in an automated way.  Automatic management of the TA is
   RECOMMENDED and can be subdivided into the following sub-categories:

   1.  Initial TA provisioning  that is the ability, for a DRO, to
          ensure a starting resolver is automatically provisioned with
          an up-to-date configuration.  This includes the TA associated
          to the trust model established by the DRO.

   2.  TA update over time  while the trust model may not evolve, the

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          cryptographic keys associated to the security entry points are
          subject to change and thus TA needs to be updated over time.
          A DRO needs to check TA updates properly.

   3.  TA reporting  The reporting of the TA used by the resolver is
          made to the DRO as well as the authoritative servers which are
          hold responsible for making their zone validated by DNSSEC

   This section is only considering TA and not NTA.  The handling of NTA
   is detailed in Section 8.

7.1.  Trust Anchor Configuration

   When a DRO starts a DNSSEC resolver, the DNSSEC resolver is
   provisioned with the TAs as part of its configuration.  As these TAs
   change over time, the DRO MUST ensures resolvers are always
   provisioned with up-to-date TAs and detect deprecated configuration.

   To do so, the TA configuration is considered as a trusted model
   instantiated as follows:

   1.  Trust model definition :The DRO defines the domain names that
       constitutes security entry point (TA) - see Section 7.1.1 for
       more details.

   2.  Trust model bootstrapping  A bootstrapping procedure is applied
          to each TA to retrieve their TA values - that is the
          corresponding value of the key - in a trusted way.  Such TA
          MAY have a specific format not understood by the resolver -
          see Section 7.1.2 for more details.

   3.  DNSSEC resolver configuration generation  The TA with associated
          values are formatted appropriately for the DNSSEC resolver
          that will be instantiated.  In many cases the appropriated
          format is the DNS RRset format - see Section 7.1.3 for more

   4.  DNSSEC resolver instantiation  The DNSSEC resolver is started,
          performs some checks before becoming operational, that is
          checking comp ability between provisioned TAs and those found
          online.  These checks are implemented by the resolver and not
          really in scope of the DRO - see Section 7.1.4 for more

   While these steps may require some some specific development for
   complex trust model, no additional deployment is required when using
   the default model where the root zone is defined as the only secure

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   entry point.  In such case, the trusted model bootstrapping is
   performed by software-update that relies on the code signing key of
   the software provider.  The software provider also provides the
   configuration to the appropriated format and the checks at
   instantiation - see Section

7.1.1.  Definition of the Trust model

   The DRO defines its trust model by explicitly mentioning the domain
   name that constitutes security entry point as well as domain name
   that are known to be unsecured.

   This document does not provide recommendations regarding the number
   of TA a DRO needs to configure its DNSSEC resolver with.  There are
   many reasons a DRO may be willing to consider multiple TAs as opposed
   to a single Root Zone Trust Anchor.  In fact it is not always
   possible to build a trusted delegation between the Root Zone and any
   sub zone.  This may happen, for example, if one of the upper zones
   does not handle the secure delegation or improperly implement it.
   Typically, a DS RRset may not be properly filled or its associated
   signature cannot be validated.  As the chain of trust between a zone
   and the root zone may not be validated, the DNSSEC validation for the
   zone requires a TA.  Such DNS(SEC) resolutions may be critical for
   infrastructure management.  A company "Example" may, for example,
   address all its devices under the domain and may not want
   disruption to happen if the .com delegation cannot be validated for
   any reasons.  Such companies may operate DNSSEC with a TA for the
   zone in addition to the regular DNSSEC delegation.
   Similarly some domains may present different views such as a
   "private" view and a "public view".  These zones may have some
   different content, and may use a different KSK for each view.

   The domain name chosen as security entry point may overlap themselves
   such as and .com for example.  The TA associated to a
   domain name is determined by the longest match.  However, the trust
   model MUST at least ensure that any domain name in the DNS be related
   to a TA.  As the number of top level domains is evolving overtime, it
   remains safe to keep the root zone as a security entry point in order
   to cover the full domain name space.

   The default trust model consists in the root zone as a security entry
   point, and no zones being considered as unsecured.

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7.1.2.  Trust Model Bootstrapping

   The purpose of the boostrapping step is clearly to securely retrieve
   DNSKEY as well as DS RRsets with a valid and authentic RDATA to
   implement the trust model (see Section 7.1.1).  Authentic data
   includes data that are up-to-date at the time these are requested,
   provided with securely and verified.  In particular the TA MUST NOT
   be retrieved from a local source that is not known to be up to date.
   This typically includes software or any local store of data for which
   there is not a dedicated and automated updating system.  Similarly,
   TA MUST NOT be retrieved from untrusted communication such as a DNS
   resolution that cannot be verified or validated.

   The authenticity of the RDATA is usually based on the authentication
   of the source which can take multiple forms, but the principle is
   that a chain of signature ends up being validated by a trusted key.
   For TA that are not the root zone KSK, DNSSEC may be used to retrieve
   and validate the TA.  Note that such means to validate the
   authenticity, the TA as a subdomain mostly prevents potential
   disruptions of the parent domains.  In many cases, an alternative
   trusted source will be preferred such as TLS which is likely to rely
   on the Web PKI, or the signature of a file.

   Although some bootstrapping mechanisms to securely retrieve publish
   [RFC7958] and retrieve [UNBOUND-ANCHOR] the Root Zone Trust Anchor
   have been defined, it is believed these mechanisms should be extended
   to other KSKs or Trust Anchors.  Such bootstrapping process enables a
   DRO to start a DNSSEC resolver from a configuration file, that
   reflects the trust model of the DRO.


   *  DRO SHOULD only rely on TA associated with a bootstrapping
      mechanism.  IANA Trust Anchor Bootstrapping

   For validators that may be used on the global public Internet (with
   "may be" referring to general purpose, general release code),
   handling the IANA managed root zone KSK trust anchor is a

   The IANA managed root zone KSK is an operationally significant trust
   point in the global public Internet.  Attention to the trust anchor
   for this point is paramount.  Trust anchor management ought to
   recognize that the majority of operators deploying DNSSEC validators
   will need to explicitly or implicitly rely on this trust anchor.
   Trust anchor management needs to recognize that there may be other

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   trust anchors of interest to operators.  Besides deployments in
   networks other than the global public Internet (hence a different
   root), operators may want to configure other trust points.

   The IANA managed root zone KSK is managed and published as described
   in "DNSSEC Trust Anchor Publication for the Root Zone" [RFC7598].
   That document is written as specific to that trust point.  Other
   trust points may adopt the technique describe (or may use other

   This represents a consideration for implementations.  On one hand,
   operators will place special emphasis on how the root zone DNSSEC KSK
   is managed.  On the other hand, implementations ought to accommodate
   trust anchors in a general manner, despite the odds that other trust
   anchors will not be configured in a specific deployment.

   Because of this, it is recommended that implementations make the root
   zone trust anchor obvious to the operator while still enabling
   configuration of general trust points.

7.1.3.  Configuration Generation

   The generation of a configuration file associated to the TA is
   expected to be implementation independent.  The necessity of tweaking
   the data depending of the software implementer or eventually the
   software version introduces a vector for configuration errors.

7.1.4.  DNSSEC Resolver Instantiation


   *  DNS resolver MUST validate the TA before starting the DNSSEC
      resolver, and a failure of TA validity check MUST prevent the
      DNSSEC resolver to be started.  Validation of the TA includes
      coherence between out-out band values, values stored in the DNS as
      well as corresponding DS RRsets.

7.2.  Trust Anchor Update

   Updating the TA reflects the evolution of the trust.  It is important
   to understand that this section does not consider the trust model to
   be updated by the DRO.  This has been defined in section Section 7.1.
   Instead, it considers the evolution over time of the instantiation of
   the trust model, that is the update of the values associated to the
   TA.  Such updates need to be operated in a reliable and trusted way.

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   The value associated to the TA may be updated over time which is part
   of the maintenance of the configuration and needs to be performed by
   the DNSSEC resolver without any intervention of the DRO.  This is the
   purpose of this section.

   TA update is expected to be transparent to the DRO (see
   Section 7.2.1.  However, a DRO MAY wish to ensure its resolvers
   operate according to the provisioned configurations and are updated
   normally (see Section 7.2.2.  This includes for a DRO the ability to
   check which TA are in used as well as to resolve in collaboration of
   authoritative servers and report the used TAs.

7.2.1.  Automated Updates to DNSSEC Trust Anchors

   Trust is inherently a matter of an operations policy.  As such, a DRO
   will need to be able to update the list of Trust Anchors.  TA updates
   are not expected to be handled manually.  This introduces a
   potentially huge vector for configuration errors, due to human
   intervention as well as potential misunderstanding of ongoing


   *  DRO SHOULD enable "Automated Updates to DNSSEC Trust Anchors"
      [RFC5011] [I-D.ietf-dnsop-rfc5011-security-considerations].

7.2.2.  Automated Trust Anchor Check

   A DRO SHOULD regularly check the trust anchor used by the DNSSEC
   resolver is up-to-date and that values used by the resolvers are
   conform to the ones in the configuration (see Section 7.1).  Such
   check is designated as TA health check.

   Note that retrieving in an automated way the value of the TA removes
   old values from the configuration and ensures that resolvers are
   always started with up-to-date values.  In the case of a key roll
   over, the resolver is moving from an old value to an up-to date
   value.  This up-to-date value does not need to survive reboot, and
   there is no need to update the configuration file of the running
   instances - configuration is updated by a separate process.  To put
   it in other words, the updated value of the TA is only expected to be
   stored in the resolver's memory.  Avoiding the configuration file to
   be updated prevents old configuration file to survive to writing
   error on read-only file systems.

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   The TA used by a resolver may be part of a configuration parameter as
   well as part of an internal state of the resolver.  It is NOT
   RECOMMENDED a DRO accesses configuration or internal state of a
   resolver as it may open the resolver to other vulnerabilities and
   provides privileged access to a potential attacker.


   *  DRO SHOULD enable "Signaling Trust Anchor Knowledge in DNS
      Security Extensions (DNSSEC)" [RFC8145] to provide visibility to
      the TA used by the resolver.  The TA can be queried using a DNSKEY
      query.  The channel MAY be protected and restricted to the DRO.

   Note also that [RFC8145] does not only concern Trust Anchor but is
   instead generic to DNSKEY RRsets.  As a result, unless for the root
   zone, it is not possible to determine if the KSK/ZSK or DS is a Trust
   Anchor or a KSK/ZSK obtained from regular DNSSEC resolutions.

   TA health check includes validating DNSKEY RRsets and associated DS
   RRsets in the resolver, on the DNS authoritative servers as well as
   those obtained out-of-band.  TA health check results MUST be logged.
   The check SHOULD evaluate if the mismatch resulted from an ongoing
   normal roll over, a potential emergency key roll over, failed roll
   over or any other envisioned cases.  Conflicts are not inherently a
   problem as some keys may be withheld from distribution via the DNS.
   A failed key roll over or any other abnormal situation MUST trigger
   an alarm.


   *  A DRO SHOULD regularly run TA health checks.

   If the mismatch is due to a failed key roll-over, this SHOULD be
   considered as a bug by the DRO.  The DRO MUST restart the resolver
   with updated TA.


   *  A DRO SHOULD be able to check the status of a TA as defined in
      Section 3 of [RFC7583].

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8.  Negative Trust Anchors Related Recommendations

   When the DNSSEC Resolver is not able to validate signatures because a
   key or DS has been published with an error, the DNSSEC Operator MAY
   temporarily disable the signature check for that key until the time
   the error is addressed.  This is performed using NTA[RFC7646].  NTA
   represents the only permitted intervention in the resolving process
   for a DRO.

   NTA are considered as temporary fix for a known unsecured domain,
   which is different from an TA that would not be trusted.  The
   designation of NTA might be misleading, but NTA are not expected to
   be part of the trust model.  This does not prevent a DRO to provision
   NTA as a configuration parameters NTA.  The management of the
   configuration SHOULD be automated as described in Section 7.1.

   Handling NTA is described in [RFC7646] and a DRO SHOULD follow these
   guidelines.  The intent of this section is to position these
   guidelines toward the operational recommendations provided in this


   *  DRO SHOULD be able to automatically configure NTA when starting
      DNSSEC resolvers.


   *  DRO SHOULD set automated procedures to determine the NTA of DNSSEC

   *  DRO SHOULD be able to handle NTA as defined in [RFC7646].

   Note that adding a Negative Trust Anchor only requires the domain
   name to be specified.  Note also that NTA can disable any sort of
   DNSKEY and is not restricted to TA.

   A failure in signaling validation is associated to a mismatch between
   the key and the signature.  DNSKEY/DS RRsets for TA have a higher
   level of trust then regular KSK/ZSK.  In addition, DRO are likely to
   have specific communication channel with TA maintainer which eases
   trouble shooting.

   A signature validation failure is either an attack or a failure in
   the signing operation on the authoritative servers.  The DRO is
   expected to confirm this off line before introducing the NTA.  This
   is likely to happen via a human confirmation.  As a result here are
   the following recommendations:

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   *  DRO SHOULD monitor the number of signature failure associated to
      each DNSKEY.  These number are only hints and MUST NOT trigger
      automated insertion of NTA.

   *  A DRO MAY collect additional information associated each DNSKEY
      RRSets.  This information may be useful to follow-up roll over
      when these happen and evaluate when a key roll over is not
      performed appropriately on the resolver side or on the
      authoritative server.  It would provide some means to the DRO to
      take action with full knowledge without necessary asking for a
      confirmation.  In other cases it could prevent invalidation to
      happen.  These check may be performed for a limited subset of
      domains or generalized.

9.  ZSK / KSK (non TA) Related Recommendations

   KSK / ZSK are not part of the DNSSEC validator configuration.  Their
   values in the DNS Caches may not reflect those published by the
   authoritative servers or may be incoherent with the RRset in the DNS
   Cache they are validating.  However, such incoherence primary results
   from error in the management of the authoritative servers.  As a
   result, it is not expected that the DNSSEC validator provides complex
   management facilities to address these issues as this will modify the
   DNS architecture and add complexity that is not proved to be
   beneficial.  As a result, recommendations always belong to the run
   time or on demand recommendations.  The main difference between TA
   and KSK/ZSK is that the DRO does not necessarily have an out of band
   mechanism to retrieve the RRsets.  As a result, the DRO has less
   information to determine and confirm what is happening.  The default
   recommendation is to let things go.

   A number of reasons may result in inconsistencies between the RRsets
   stored in the cache and those published by the authoritative server.

   An emergency KSK / ZSK rollover may result in a new KSK / ZSK with
   associated new RRSIG published in the authoritative zone, while
   DNSSEC validator may still cache the old value of the ZSK / KSK.  For
   a RRset not cached, the DNSSEC validator performs a DNSSEC query to
   the authoritative server that returns the RRset signed with the new
   KSK / ZSK.  The DNSSEC validator may not be able to retrieve the new
   KSK / ZSK while being unable to validate the signature with the old
   KSK / ZSK.  This either results in a bogus resolution or in an
   invalid signature check.  Note that by comparing the Key Tag Fields,
   the DNSSEC validator is able to notice the new KSK / ZSK used for
   signing differs from the one used to generate the received generated
   signature.  However, the DNSSEC validator is not expected to retrieve

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   the new ZSK / KSK, as such behavior could be used by an attacker.
   Instead, ZSK / KSK key roll over procedures are expected to avoid
   such inconsistencies.

   Similarly, a KSK / ZSK roll over may be performed normally, that is
   as described in [RFC6781] and [RFC7583].  While the KSK / ZSK roll
   over is performed, there is no obligation to flush the RRsets in the
   cache that have been associated with the old key.  In fact, these
   RRsets may still be considered as trusted and be removed from the
   cache as their TTL timeout.  With very long TTL, these RRsets may
   remain in the cache while the ZSK / KSK with a shorter TTL is no
   longer published nor in the cache.  In such situations, the purpose
   of the KSK / ZSK used to validate the data is considered trusted at
   the time it enters the cache, and such trust may remain after the KSK
   / ZSK is being rolled over.  Note also that even though the data may
   not be associated to the KSK / ZSK that has been used to validate the
   data, the link between the KSK / ZSK and the data is still stored in
   the cache using the RRSIG.  Note also that inconsistencies between
   the ZSK / KSK stored in the cache and those published on the
   authoritative server, may lead to inconsistencies to downstream
   DNSSEC validators that rely on multiple cache over time.

   Typically, a request for the KSK / ZSK may have been provided by a
   cache that is storing the new published value, while the data and
   associated signatures may be associated to the old KSK / ZSK.

   Incoherence between RRsets and DNSKEYs is not the responsibility of
   the DRO.  Instead, it is the responsibility of authoritative server
   publishing these data.  This includes insuring the coherence between
   TTLs and signature validation periods as well as small variations of
   the resolvers clocks.  Section 4.4.1 of [RFC6781] provides some
   recommendations that can be implemented by the authoritative server
   which puts the responsibility of failure of signature validation
   under the responsibility of the authoritative server.  A DRO MAY
   however limit the risks for these inconsistencies to happen by
   configuring the DNSSEC validator with generic rules that applies to
   the validation process.  Typically, the TTL associate to the DNSKEY
   is an engagement from the authoritative server that the DNSKEY will
   remain valid over this period.  As this engagement supersedes the
   validation of any RRSIG and by extension to any RRset in the zone,
   this TTL value may be used as the maximum value for the TTL
   associated to FQDNs in the zone.  Section 8.1 of [RFC4033] mention
   the ability by the resolver to set the upper bound of the TTL to the
   remaining signature validity period.  This would at least reduce
   inconsistencies during regular KSK roll over.  In addition, the
   DNSSEC validator should also be able to provide a maximum values for
   TTLs.  These values MAY also consider the small inaccuracy of the
   local clock.

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   *  To limit the risks of incoherent data in the cache, it is
      RECOMMENDED DRO enforce TTL policies of RRsets based on the TTL of
      the DS, KSK and ZSK.  RRsets TTL SHOULD NOT exceed the DS, KSK or
      ZSK initial TTL value, that TTL SHOULD trigger delegation
      revalidation as described in [I-D.ietf-dnsop-ns-revalidation].
      TTL SHOULD NOT exceed the signature validity.

10.  DNSKEY Related Recommendations

   This section considers the recommendations that are common to TA as
   well as non TA DNSKEY RRsets.

10.1.  Automated Reporting

   A DRO MAY regularly report the Trust Anchor used to the authoritative
   server.  This would at least provide insight to the authoritative
   server and provide some context before moving a key roll over

   The purpose of reporting the currently used Trust Anchor for a domain
   name is to establish an informational channel between the resolver
   and the authoritative server.  This data may not directly be useful
   for the DNSSEC Resolver, but instead to the authoritative server.  In
   return it is likely the authoritative server will take the
   appropriate steps in operating the authoritative server and consider
   this information.  This results in the following recommendation:


   *  A DRO SHOULD enable TA reporting to the authoritative server as
      specified in "Signaling Trust Anchor Knowledge in DNS Security
      Extensions (DNSSEC)" [RFC8145]

10.2.  Interactions with the cached RRsets

   The purpose of automated checks is to enable early detection of
   failed operations, which provides enough time to the DRO to react
   without any consequences.  On the other hand, these checks MAY reveal
   as well that a rogue TA has been placed and that the resolver is
   corrupted.  Similarly, a DRO may be informed by other channel a rogue
   or unwilling DNSKEY has been emitted.

   In such situation, the DRO SHOULD be able to remove the RRsets
   validated by the rogue DNSKEY.


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   *  A DRO MUST be able to flush the cached data associated to a DNSKEY

11.  Cryptography Deprecation Recommendations

   As mentioned in [RFC8247] and [RFC8221] cryptography used one day is
   expected over the time to be replaced by new and more robust
   cryptographic mechanisms.  In the case of DNSSEC signature protocols
   are likely to be updated over time.  In order to anticipate the
   sunset of one of the signature scheme, a DNSSEC validator may be
   willing to estimate the impact of deprecating one signature scheme.

   Currently [RFC6975] provides the ability for a DNSSEC validator to
   announce an authoritative server the supported signature schemes.
   However, a DNSSEC validator is not able to determine other than by
   requesting and monitoring DNSKEY RRsets as well as RRSIG.  These
   RRsets are received by enabling DNSSEC validation by default which is
   obviously the case for DNSSEC validator.

   To safely deprecate one signature scheme, the DNSSEC validator
   operator is expected to follow the recommendation below:


   *  A DRO SHOULD regularly request and monitor the signature scheme
      supported by an authoritative server.

   *  A DRO SHOULD report a "Unsupported DNSKEY Algorithm" as defined in
      [RFC8914] when a deprecated algorithm is used for validation.

12.  Invalid Reporting Recommendations

   A DNSSEC validator receiving a DNS response cannot make the
   difference between receiving an non-secure response versus an attack.
   Dropping DNSSEC fields by a misconfigured middle boxes, such as DS,
   RRRSIG is considered as an attack.  A DNSSEC validator is expected to
   perform secure DNS resolution and as such protects its stub client.
   An invalid response may be the result of an attack or a
   misconfiguration, and the DNSSEC validator may play an important role
   in sharing this information with the authoritative server or domain
   name owner.


   *  DRO SHOULD monitor and report DNSSEC validation error.  Reporting
      may take various means but the DRO SHOULD implement [RFC8914] to
      inform the DNS client.

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13.  IANA Considerations

   There are no IANA consideration for this document.

14.  Security Considerations

   The recommendations listed in this document have two goals.  First
   ensuring the DNSSEC validator has appropriated information to
   appropriately perform DNSSEC validation.  Second, monitoring the
   necessary elements that would enable a DNSSEC validator operator to
   ease a potential analysis.  The recommendations provide very limited
   ability for a DNSSEC validator operator to alter or directly
   interfere on the validation process and the main purpose in providing
   the recommendations was to let the protocol run as much as possible.
   Providing inappropriate information can lead to misconfiguring the
   DNSSEC validator, and thus disrupting the DNSSEC resolution service.
   As a result, enabling the setting of configuration parameters by a
   third party may open a wide surface of attacks.  In addition, such
   changes may lead to unexpected corner cases that would result in
   making analysis and trouble shooting very hard.

   As an appropriate time value is necessary to perform signature check,
   an attacker may provide rogue time value to prevent the DNSSEC
   validator to check signatures.

   An attacker may also affect the resolution service by regularly
   asking the DNSSEC validator to flush the KSK/ZSK from its cache.  All
   associated data will also be flushed.  This generates additional
   DNSSEC resolution and additional validations, as RRSet that were
   cached require a DNSSEC resolution over the Internet.  This affects
   the resolution service by slowing down responses, and increases the
   load on the DNSSEC validator.

   An attacker may ask the DNSSEC validator to consider a rogue KSK/ZSK,
   thus hijacking the DNS zone.  Similarly, an attacker may inform the
   DNSSEC validator not to trust a given KSK in order to prevent DNSSEC
   validation to be performed.

   An attacker (cf.  Section 7) can advertise a "known insecure" KSK or
   ZSK is "back to secure" to prevent signature check to be performed

   As a result, information considered by the DNSSEC validator should be
   from a trusted party.  This trust party should have been
   authenticated, and the channel used to exchange the information
   should also be protected and authenticated.

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   The software used for DNSSEC validator is not immune to bugs and may
   become vulnerable independently of how it is operated.  As a result a
   DRO SHOULD NOT depend on a single implementation or version of a
   given software and SHOULD instead run at least two independent pieces
   of software.

15.  Contributors

   We would like to thank very much Edward Lewis without who the
   document might probably not exist.

16.  Acknowledgment

   The need to address DNSSEC issues on the resolver occured during
   multiple discussions including among others Ted Lemon, Ralph Weber,
   Normen Kowalewski, Mikael Abrahamsson, Jim Gettys, Paul Wouters, Joe
   Abley and Michael Richardson.

17.  References

17.1.  Normative References

              Huque, S., Vixie, P., and R. Dolmans, "Delegation
              Revalidation by DNS Resolvers", Work in Progress,
              Internet-Draft, draft-ietf-dnsop-ns-revalidation-02, 7
              March 2022, <

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,

   [RFC5011]  StJohns, M., "Automated Updates of DNS Security (DNSSEC)
              Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011,
              September 2007, <>.

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   [RFC6781]  Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
              Operational Practices, Version 2", RFC 6781,
              DOI 10.17487/RFC6781, December 2012,

   [RFC6975]  Crocker, S. and S. Rose, "Signaling Cryptographic
              Algorithm Understanding in DNS Security Extensions
              (DNSSEC)", RFC 6975, DOI 10.17487/RFC6975, July 2013,

   [RFC7583]  Morris, S., Ihren, J., Dickinson, J., and W. Mekking,
              "DNSSEC Key Rollover Timing Considerations", RFC 7583,
              DOI 10.17487/RFC7583, October 2015,

   [RFC7646]  Ebersman, P., Kumari, W., Griffiths, C., Livingood, J.,
              and R. Weber, "Definition and Use of DNSSEC Negative Trust
              Anchors", RFC 7646, DOI 10.17487/RFC7646, September 2015,

   [RFC8145]  Wessels, D., Kumari, W., and P. Hoffman, "Signaling Trust
              Anchor Knowledge in DNS Security Extensions (DNSSEC)",
              RFC 8145, DOI 10.17487/RFC8145, April 2017,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8221]  Wouters, P., Migault, D., Mattsson, J., Nir, Y., and T.
              Kivinen, "Cryptographic Algorithm Implementation
              Requirements and Usage Guidance for Encapsulating Security
              Payload (ESP) and Authentication Header (AH)", RFC 8221,
              DOI 10.17487/RFC8221, October 2017,

   [RFC8247]  Nir, Y., Kivinen, T., Wouters, P., and D. Migault,
              "Algorithm Implementation Requirements and Usage Guidance
              for the Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 8247, DOI 10.17487/RFC8247, September 2017,

   [RFC8914]  Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
              Lawrence, "Extended DNS Errors", RFC 8914,
              DOI 10.17487/RFC8914, October 2020,

17.2.  Informative References

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   [ENT]      Levigneron, V., "ENT was here !!!", n.d.,

              Hardaker, W. and W. Kumari, "Security Considerations for
              RFC5011 Publishers", Work in Progress, Internet-Draft,
              draft-ietf-dnsop-rfc5011-security-considerations-13, 16
              July 2018, <

   [RFC7598]  Mrugalski, T., Troan, O., Farrer, I., Perreault, S., Dec,
              W., Bao, C., Yeh, L., and X. Deng, "DHCPv6 Options for
              Configuration of Softwire Address and Port-Mapped
              Clients", RFC 7598, DOI 10.17487/RFC7598, July 2015,

   [RFC7958]  Abley, J., Schlyter, J., Bailey, G., and P. Hoffman,
              "DNSSEC Trust Anchor Publication for the Root Zone",
              RFC 7958, DOI 10.17487/RFC7958, August 2016,

              "unbound-anchor - Unbound anchor utility", n.d.,

Authors' Addresses

   Daniel Migault
   8275 Trans Canada Route
   Saint Laurent, QC  4S 0B6

   Dan York

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